It is here argued that Locke and Newton held very similar views on the nature of our knowledge of substance: our only cognitive access to substances is through their powers to affect our minds and other substances. However, in spite of this shared empiricist foundation, Locke and Newton held divergent views on the unification of powers or qualities into a single substance. While Locke allows that distinct powers can be understood as united in one substance (indeed all substances are collections of powers for Locke), this paper argues for an interpretation of Newton according to which substance cannot have distinct powers. Rather, a substance just is a power in space and time.